The present invention relates to a control for a continuously variable transmission.
European Patent Application No. 83104182.7 published under publication No. 0092228 on Oct. 26, 1983 (corresponding to U.S. Pat. No. 4,597,308 issued to Tanaka et al. on July 1, 1986, our ref.: EPI35-82) discloses a system for controlling a reduction ratio of a continuously variable transmission of an automotive vehicle having an internal combustion engine. The continuously variable transmission has a V-belt running over a driver or input pulley and a driven or output pulley, a shift actuator, in the form of a stepper motor, movable to a plurality of operating positions, a source of hydraulic fluid pressure, a valve actuated by the shift actuator for regulating fluid supply to and fluid discharge from at least one of the driver pulley and driven pulley so as to establish a reduction ratio that is predetermined for one of the plurality of operating positions assumed by the shift actuator. Sensors are provided to detect operating conditions of the vehicle, including engine speed, throttle, and vehicle speed. The system includes a computerized control which carries out a method comprising the steps of:
determining a target reduction ratio to be established in the continuously variable transmission by table look-up of a table data based on throttle opening degree and vehicle speed;
determining a target pulse number indicative of a target operating position to be assumed by the shift actuator in order to establish the target reduction ratio;
generating as a parameter an actual pulse number that is set to zero when the largest reduction ratio is established in the continuously variable transmission;
varying the actual pulse number in such a direction as to reduce a difference between the target pulse number and actual pulse number; and
moving the shift actuator toward the target operating position thereof in response to a variation in the actual pulse number.
The table data contains a zero target pulse number at a location having an address of zero vehicle speed for ease of moving the vehicle from a standstill so that the largest reduction ratio is established whenever the vehicle speed indicative signal is zero. In order to maintain one-to-one correspondance between the actual pulse number and the actual operating position of the shift actuator with good accuracy, the actual pulse number is set to zero whenever a shift reference switch is turned ON by the shift actuator upon assuming a predetermined operating position at which the largest reduction ratio is established in the continuously variable transmission. This known system contains a potential problem that a zero target pulse number is issued if the vehicle speed indicative signal becomes zero due to failure or trouble of the vehicle speed sensor. If the vehicle speed indicative signal should become zero during running of the vehicle, the transmission would be shifted down toward the largest reduction ratio, subjecting the vehicle to undesirable engine brake running. This may cause the engine to overrun if the vehicle is travelling at a high speed.
JP-A No. 59-75840 published on Apr. 28, 1984 (corresponding to European Patent Application No. 83110546.5 published under publication No. 0108315 on June 25, 1986, and to U.S. Pat. No. 4,576,265 issued to Kumura et al. on Mar. 8, 1986, our ref.: EP081-83) discloses a control system designed to execute not only a reduction ratio control, but also a start-up clutch control.
JP-A No. 60-81560 published on May 9, 1985 (corresponding to European Patent application No. 84122095.9 published under publication No. 0139277 on May 2, 1985 and U.S. patent application No. 659,678 filed Oct. 11, 1984, now U.S. Pat. No. 4,670,843 our ref.: EP096-84) discloses a feed-forward and feed-back control in order to improve transient performance of a continuously variable transmission wherein a feed-forward control or a feed-back control is selected in response to the absolute value of an error signal indicative of a difference between an actual pulse number and a target pulse number and a shift command signal is produced based on the selected one of them in moving the shift actuator to control a reduction ratio in the continuously variable transmission. The feed-forward control is carried out on a feed-forward control value determined by calculation, while the feed-back control is carried out on the sum of the feed-forward control value and a feed-back control value. The feed-back control value is the sum of a proportional term and an integral term. With this known control system, if the error signal is considerably increased during a transient state initiated by a great and rapid increase in throttle opening degree, the proportional term increases accordingly with the increase in the error signal and an overshoot occurs in a driver pulley revolution speed before reaching a new target driver pulley revolution speed set by the new throttle position. This overshoot takes place immediately before the completion of the transient state. The rate of change in driver pulley revolution speed during the occurrence of the overshoot is great, causing a noticeable change in output shaft torque which change is recognized as a substantial shock immediately before completion of the transient state.
Referring to FIG. 8, if the throttle is rapidly opened to cause a rapid and great increase in throttle opening degree (TH), a target driver pulley revolution speed (TRPM) set in correspondance with the throttle opening degree (TH) increases rapidly and greatly, and differs greatly from an actual driver pulley revolution speed (Nt). Thus, the actual driver pulley revolution speed deviates to the minus from the target driver pulley revolution speed. Under this condition, in order to increase the actual driver pulley revolution speed, there is a need to move a target shift actuator operating position (ND) toward the predetermined position where the largest reduction ratio is established by decreasing the target pulse number toward zero. The amount of decrease in the target pulse number is variable with the proportional term expressed by Kp.times.e, where Kp is a proportional gain and e is an error and an integral term expressed by Ki.times..intg.e dt. If the amount of error e is great during the transient state, the proportional term is substantial, so the target shift actuator operating position varies sharply, causing the shift actuator to change its direction of movement rapidly. As a result, the actual driver pulley revolution speed (Nt) overshoots before the completion of the transient state and output shaft torque curve varies greatly.
An object of the present invention is to improve proportional and integral control for a continuously variable transmission such that the continuously variable transmission can exhibit shockless transient performance in shifting from one steady state to another steady state. A further object of the present invention is to provide an improved control which is free from the before mentioned potential problem that is caused by breakdown or failure of a vehicle speed sensor.